LU-4944 osd: support for zfs-0.6.3

LU-4944 osd: support for zfs-0.6.3

dsl_sync_task_nowait() replaces dsl_sync_task_do_nowait() and we have
to support the both versions: 0.6.2 and 0.6.3

Change-Id: I4e2e91eae40358e2cd68ab26107e51037c8fb09c
Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-on: http://review.whamcloud.com/10064
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>